--- - |2 Abstract Processing feedback from the environment is an essential function during development to adapt behavior in advantageous ways. One measure of feedback processing, the feedback negativity (FN), is an ERP observed following the presentation of feedback. Findings detailing developmental changes in the FN have been mixed, possibly due to limitations in traditional ERP measurement methods. Recent work shows that both theta and delta frequency activity contribute to the FN; utilizing time‐frequency methods to measure change in power and phase in these frequency bands may provide more accurate representation of feedback processing development in childhood and adolescence. We employ time‐frequency power and intertrial phase synchrony measures, in addition to conventional time‐domain ERP methods, to examine the development of feedback processing in the theta (4–7 Hz) and delta (.1–3 Hz) bands throughout adolescence. A sample of 54 female participants (8–17 years old) completed a gambling task while EEG was recorded. As expected, time‐domain ERP amplitudes showed no association with age. In contrast, significant effects were observed for the time‐frequency measures, with theta power decreasing with age and delta power increasing with age. For intertrial phase synchrony, delta synchrony increased with age, while age‐related changes in theta synchrony differed for gains and losses. Collectively, these findings highlight the importance of considering time‐frequency dynamics when exploring how the processing of feedback develops through late childhood and adolescence. In particular, the role of delta band activity and theta synchrony appear central to understanding age‐related changes in the neural response to feedback. - Psychophysiology, Volume 55, Issue 10, October 2018.